This invention relates to a voltage regulator suitable for use with an electrical generator such as an alternator, and in particular to a voltage regulator that provides improved control functions by determining and limiting alternator output current as well as output voltage.
A modern vehicle uses an alternator to power the vehicle's electrical system and to recharge a battery that provides standby electric power whenever the vehicle engine is not operating or when insufficient electric power is available from the alternator. The alternator includes a field winding, stator windings, and a rotating shaft that is driven through some arrangement by an engine. Rectifiers are used to convert the alternating current generated by the stator windings into direct current for battery charging. A voltage regulator senses the alternator output voltage and controls the field coil current to maintain a constant voltage according to the regulator's internal voltage reference as external electric loads are added and removed, within the limits of the alternator output power capacity. This is generally achieved by making current flow through the field winding whenever output voltage drops below the reference voltage, and stopping the flow of current through the field winding whenever the output voltage rises above the reference voltage.
The appropriate regulator reference voltage is determined by the battery charging voltage needed for the particular application, and the vehicle electrical system typically is designed to operate at this voltage. The reference voltage is often designed with temperature compensation because it is desirable for battery charging that the charging voltage decrease as battery temperature increases. Alternator output current is produced in the stator windings when the field winding is conducting current and the alternator shaft is turning. At constant voltage the alternator output current increases with shaft speed in a nonlinear relationship, and this output current raises the stator winding temperature. As stator temperature increases, the maximum alternator output current at constant voltage decreases. Automotive alternator output power rating is typically determined at an alternator shaft speed of 5000 revolutions per minute.
When a vehicle engine is operating at idle speed the alternator output power is typically below the rated alternator power. This often means that the alternator is incapable of supplying all of the electric power needs at engine idle speed, and the battery supplies the shortfall electric power. As the temperature of the engine compartment and of the alternator stator winding increases, the alternator maximum output power is further decreased. It is not generally known how quickly the battery is being discharged under such circumstances unless a current shunt is used to measure battery discharge current, or if other means are employed to measure the battery's state of charge.
A different situation occurs at arctic temperatures if preheat is used to start the engine and high electric loads are quickly applied when the alternator is still cold. The alternator output power under such circumstances can significantly exceed the maximum rated output power for a few minutes. The drive power to the alternator to meet the electrical demand may exceed either torque or drive limits for the drive mechanism between the engine and the alternator and cause drive failure. Alternator drives for high power alternators have little margin to exceed peak torque and peak drive limits that are typically based on room temperature data. Alternator output voltage, output current, and efficiency can be used to determine input power, while input power and shaft speed can be used to determine input torque. It is difficult to measure direct current without a shunt, a calibrated device that develops a voltage across its terminals proportional to the current flow through the shunt.
It is not apparent that anyone has addressed all of the above problems in an alternator or voltage regulator design. However, various systems have been proposed which touch upon some aspects of the above problems. For example, Ueda, U.S. Pat. No. 5,712,786, proposes an engine idle speed control method that employs, among other features, a map related to engine idle speed and alternator field duty cycle to determine alternator output current without using a shunt. The determination of alternator current is a feature in the overall control of an idle speed control valve to automatically regulate the idle speed of an internal combustion engine. Ueda does not measure the alternator shaft speed nor apply temperature compensation to measured data regarding alternator operation.
Vanek, et al, U.S. Pat. No. 5,559,704, proposes a method of computing alternator power based on measured values of alternator current and voltage for the purpose of determining efficiency and engine horsepower. Alternating current detectors and direct current shunts are used to measure current. The shaft speed is not considered in this governed engine speed locomotive application, and neither the ambient temperature nor the stator winding temperature are measured.